Integrated circuit chip manufacturers fabricate semiconductor devices by different combinations of fabrication processes. Wafer temperature is an important parameter for many of these processes. More specifically, precise measurement and control of wafer temperature and its uniformity are required in order to minimize deviations from the target process parameters and increase device fabrication yield. Thermal fabrication processes, such as thermal anneals, oxidation, and chemical vapor deposition (CVD) are examples of processes where wafer temperature is an important process parameter. Currently, thermocouples are used to measure wafer temperature during plasma etch operations such as reactive ion etch (RIE). Additionally, some chemical vapor deposition processes, such as plasma-enhanced chemical vapor deposition (PECVD) employ thermocouples for temperature measurement.
Thermocouples for measuring temperature in semiconductor fabrication equipment have substantial disadvantages. For instance, thermocouples can be disturbed by the RF and electromagnetic fields (e.g., 13.56 MHz RF and 2.45 GHz microwave) used to generate plasma during plasma etch processing. Thermocouples are also invasive in that they must be placed very near the location where the temperature is being measured. In some applications the thermocouples may require actual contact with the wafer surface for accurate temperature sensing, thereby causing disturbance of the wafer temperature and also possible wafer contamination. Furthermore, most thermocouples suffer from measurement error and slow response time problems.
Pyrometry providing for non-contact operation can also be used to measure wafer temperature in some thermal processing applications such as rapid thermal processing (RTP). Computational pyrometry sensors, however, have several disadvantages. For example, for accurate pyrometry-based temperature measurements, an accurate knowledge of the spectral emissivity of the wafer at the pyrometry measurement wavelength band is required. It is known that spectral emissivity can vary with various parameters, such as wafer temperature, wafer resistivity, material layers, and their thickness, and process chamber geometry and materials, making pyrometry sensors relatively inaccurate due to emissivity variations and other noise sources. In general, conventional pyrometry techniques can suffer from measurement inaccuracies and repeatability errors as high as .+-.100.degree. C. or more. Moreover, pyrometry techniques require frequent cross-calibrations using thermocouples introducing the problems associated with thermocouples noted previously.